Recently, liquid crystal display devices have been widely used as a visible image displaying portion of various types of electronic equipment such as a video camera and the like. The liquid crystal display device modulates light by controlling the alignment of a liquid crystal depending upon whether a prescribed voltage is imposed thereon or not to thereby display a visible image such a character, a numeral and the like.
In general, the liquid crystal display device is arranged such that one pixel is formed by a liquid crystal partitioned to a dot shape and a visible image display region is formed by disposing a plurality of the pixels to a matrix shape. Then, the visible image is displayed making use of contrast of light produced depending upon whether a prescribed voltage is imposed or not on each of the respective liquid crystals which form the respective pixels. To obtain optimum contrast in this case, the voltage imposed on each liquid crystal must be maintained to a prescribed value which is optimum to the liquid crystal.
In an ordinary liquid crystal display device, a liquid crystal driving IC is mounted on a liquid crystal panel and when an output voltage of a host equipment side, for example, a portable telephone, is imposed on the external input terminal of the liquid crystal driving IC, a voltage which will be imposed on a liquid crystal appears to the output terminal of the liquid crystal driving IC. However, the characteristics of liquid crystal driving ICs may be dispersed depending upon conditions under which they are manufactured and the characteristics of liquid crystal panels may be also dispersed. The following two methods are contemplated to impose an optimum voltage on each of the liquid crystals forming respective pixels even if such dispersion arises.
First, there is contemplated a method of adjusting a voltage imposed on a liquid crystal to a prescribed value by adjusting a voltage supplied from host equipment to a liquid crystal driving IC. Another method is such that a resistance element is additionally connected to the liquid crystal driving IC and a voltage imposed on the liquid crystal is adjusted to a prescribed value by adjusting the resistance value of the resistance element while maintaining the voltage supplied from the host equipment to a given value.
Taking into consideration an actual situation that a liquid crystal display device is finished as a product in a state that a liquid crystal driving IC is mounted on a liquid crystal panel, it is preferable that the output voltage of host equipment supplied to the liquid crystal display device is adjustable. Actually, however, the method in which the adjusting of the voltage is performed in the liquid crystal display device while maintaining the voltage supplied from the host equipment to the given value has been widely used.
Conventionally, when a voltage is not adjusted on a host equipment side but adjusted on a liquid crystal display device side, an FPC (flexible printed circuit) or a transit PCB (printed circuit board) is connected to the external connecting terminal of a liquid crystal panel and a resistance element is mounted on the FPC or the transit PCB and an operating voltage which will be supplied to a liquid crystal driving IC is changed by the action of the resistance element.
However, the conventional liquid crystal display device having the resistance element mounted on the FPC or the transit PCB has a problem that an additional area or space is required on the FPC or the like to mount the resistance element thereon and, as a result, the liquid crystal display device is increased in size accordingly.
In particular, when there is taken into consideration a liquid crystal display device of a type having a liquid crystal driving IC(s) directly mounted on one or both of a pair of light transmitting substrates having a liquid crystal sandwiched therebetween, that is, a liquid crystal display device of a COG (chip on glass) system, the liquid crystal display device has a great advantage that it does not especially require a large FPC, transit PCB and the like because the liquid crystal display device equips the liquid crystal driving IC(s) on the transparent substrate(s). Therefore, concerning the liquid crystal display device of the COG type, in case the FPC or the transit PCB is additionally mounted on the liquid crystal display device wittingly in order to add a resistance element, the advantage of the liquid crystal display device is greatly reduced by it.
Incidentally, what is additionally connected to the liquid crystal driving IC is not limited to the resistance element and there is a case that a peripheral circuit including a resistance element, a capacitor and the like is disposed on the liquid crystal driving IC to assist the operation thereof. The capacitor is provided because of the following two reasons.
(1) Voltages imposed on respective liquid crystals corresponding to respective pixels in a liquid crystal panel appear to the output terminals of a liquid crystal driving IC. In the ordinary liquid crystal display devices, in order to impose desired voltages on only the liquid crystals of desired pixels and not to impose a voltage on the liquid crystals of the pixels disposed around the above pixels, it is insufficient only to impose a single pulse voltage on a scanning electrode and a signal electrode, and multi-stage pulse voltages must be properly superimposed and imposed thereon. To impose the multi-stage pulse voltages on the respective pixels as described above, the multi-stage pulse voltages must be individually output to the output terminals of the liquid crystal driving IC. In this case, if the respective pulse voltages are unstable, there is a possibility that noise arises and the quality of display of the liquid crystal display device is adversely affected by the noise. To solve this problem, capacitors are connected in parallel between respective voltage levels from which the multi-stage pulse voltage outputs are originated.
(2) A general liquid crystal driving IC needs VDD=+3V to drive a logic system circuit and Vss=GND for grounding and further VLCD=−6V to drive a liquid crystal display device. Depending upon a type of the liquid crystal driving IC, there is a liquid crystal driving IC provided with three input terminals separately for the three potentials or a liquid crystal driving IC of a system which is provided with only input terminals corresponding to VSS=GND and VDD=+3V and creates VLCD=−6V in the interior thereof. When the liquid crystal driving IC of the system for creating VCLD from VSS and VDD is used, a voltage increasing circuit including a capacitor must be additionally provided as a peripheral circuit around the liquid crystal driving IC.
Conventionally, when a peripheral circuit including a resistance element and a capacitor is additionally provided with a liquid crystal driving IC, the FPC or a transit PCB is connected to a liquid crystal panel and the peripheral circuit is amounted on the transit PCB or the like. Further, Japanese Laid-open No. 8-43845 and Japanese Laid-Open No. 8-43846 disclose a technology for directly mounting a capacitor on a substrate of a liquid crystal display device.
However, when the peripheral circuit is mounted on the transit PCB or the like, the area of the transit PCB or the like must be increased accordingly and further a large space must be prepared. Thus, there is a problem that the overall dimension of the liquid crystal display device is increased. In addition, a job for assembling the peripheral circuit on the transit PCB or the like is troublesome.
Further, when the peripheral circuit is directly mounted on the substrate of the liquid crystal display device as disclosed in Japanese Laid-open No. 8-43845 and the like, since the area of the projecting portion of the substrate must be increased accordingly, there is also a problem that the overall dimension of the liquid crystal display device is increased.
An object of the present invention is made in view of the above problems arisen when a resistance element is additionally connected to a liquid crystal driving IC and also the above problem arisen when a peripheral circuit including a capacitor and the like is additionally connected to a liquid crystal driving IC. It is the object of the present invention to maintain a small overall dimension of a liquid crystal display device even if a resistance element is additionally connected to a liquid crystal driving IC or even if a peripheral circuit including a capacitor and the like is additionally connected to the liquid crystal driving